Leg sweep definitely works. We had our Monk sweep one (or two both sometimes depending on where they charged) and our DK Tank Death Grip the other after he starts casting it. I wouldn't recommend stunning mid-charge, it always seemed to bug and he'd finish the charge and cast it anyway; we had limited stuns.

You actually have to stun him during the cast or he'll continue the charge. For example if I clash and he doesn't get stunned he continues the charge. If I clash after he starts channeling I'll interrupt him 100% of the time.

What worked for us: stuns whenever (during charge or during channel) and knockbacks or grips once the channel starts (not during the charge).

Interrupts/silences don't work. Disorients should work if they don't immune it. I saw our mage was using Dragon's Breath, but pretty much everyone in our raid has a stun or knockback, so I'm not sure what actually stopped the channel.

Stuns, knockbacks, AND taunts all worked for us. We had our holy paladin and resto druid watching them and they never got any ticks off at all. All were used immediately once the channel started. Just assign a couple people to it and it's a non issue.